Ultrasound is widely used in medicine and industry. Example applications include imaging to view internal structures of a patient or an industrial apparatus, and measurement, for instance measuring the size or movement of internal structures. An ultrasound generator uses a transducer to convert an electrical drive signal into ultrasound pressure waves. The ultrasound pressure waves propagate through a medium, for instance human tissue, and reflect back towards the transducer when encountering an impedance mismatch. The reflected pressure waves are converted back to electrical signals by the transducer. In an ultrasound imaging system the converted electrical signals are used to form an image.
An ultrasound generator may comprise a single transducer which is a single source of ultrasound pressure waves. However, ultrasound generators often contain an array of transducer elements. Each transducer element requires a transmitter circuit. For an array of transducer elements, multiple transmitter circuits are required if each element is to be separately drivable. Transmitter circuits typically require a combination of high power and high voltage. Excitation with arbitrary analogue waveforms requires the use of arbitrary waveform generators and high power precision amplifiers. High power and high voltage switched mode excitation can be achieved using Metal Oxide Semiconductor Field-Effect Transistors (MOSFET) based transmitter circuits. Multiple MOSFETs and their associated drive electronics can be combined within a single integrated circuit package to supply high currents to an ultrasound transducer element (in the form of a piezoelectric load), reducing component count and minimising cost per excitation channel. MOSFET based transmitter circuits use switched excitation to select between several positive and negative voltage levels. Switched excitation is well suited to portable systems and phased ultrasound transducer arrays, where size, complexity and cost are critical. Switched excitation results in square pulsed signals or staircase (stepped) pulsed signals which switch between discrete levels to approximate ideal sinusoidal signals. In the present patent specification the term “pulsed signal” is taken to include both square wave signals and stepped signals.
Advances in areas of ultrasound including high frequency imaging and a requirement for portable, low-cost systems, increases the complexity of ultrasound transmitter circuits. This problem is compounded by a trend towards the integration of both transmitter circuits and transducers into an ultrasound probe. Such integration is desirable because it improves impedance matching and reduces the size of a cable bundle between the ultrasound drive signal generator and the probe.
A limitation of MOSFET switched excitation is the use of fixed DC levels, which often results in fixed amplitude output. While it is possible to adjust switching levels in between drive signal pulses, it is desirable to be able to directly control ultrasound output pressure through the selection of the drive signal. Amplitude control is of particular importance for medical ultrasound applications, including for therapeutic and diagnostic ultrasound.
It is desirable to be able to control the form and properties of the ultrasound output pressure by adjusting the drive signal. To provide this control, it is known to use Pulse Width Modulation (PWM) techniques to adjust the drive signal. There is a continuing need to provide enhanced techniques for generating ultrasound transducer drive signals in order to provide enhanced control of ultrasound output pressure.
It is known that wide band drive signals, for instance an impulse or a small number of pulses, provides good axial resolution for reflected ultrasound signals at the expense of poor penetration. In contrast, a narrow band signal, for instance a longer duration pulse train, increases the penetration of ultrasound at the expense of reduced axial resolution. In order to increase the axial resolution for narrow band signals, it is known to use coded or frequency modulated drive signals, for instance a frequency coded pulse train. In particular, it is known to provide a linear frequency coded pulse train, in which the frequency is increased or decreased linearly over time. Such a Linear Frequency Modulated (LFM) drive signal is known as a linear chirp. A coded signal can be recovered using well known signal processing techniques, which will not be described here. However, it is difficult to accurately produce analogue chirp modulating signals through conventional PWM techniques due to poor correlation of analogue pulse cycles and drive signal pulses.
It is further known to provide pulse shaping for a LFM or other coded ultrasound drive signal, for instance by applying a standard windowing technique over the time duration of the drive signal, for instance a Hann window (which tapers the start and end of the drive signal). Such pulse shaping advantageously reduces sideband noise in the received ultrasound signal.
It is known for PWM in other fields, for instance power electronics, to be modified to reduce the Total Harmonic Distortion (THD) of the pulsed signal. However, there has been little progress towards satisfactorily reducing harmonic distortion for pulsed drive signals in ultrasonics.
“Quinary excitation method for pulse compression ultrasound measurements”, Cowell and Freear, Ultrasonics 48 (2008), 98-108, Elsevier proposes the generation of a switched excitation method for linear frequency coded excitation of ultrasonic transducers in pulse compression systems. Pulse compression sidelobes are reduced through the use of amplitude tapering at the beginning and end of the excitation signal. Amplitude tapering is achieved by the use of intermediate voltage switching levels, half of the main excitation voltages. The excitation signal is generated from an LFM analogue signal by applying multiple switching levels through use of a multi-level MOSFET circuit.
“Harmonic Cancellation in Switched Mode Linear Frequency Modulated (LFM) Excitation of Ultrasound Arrays”, Cowell et al., Ultrasonics Symposium (IUS), 2011 IEEE International, pp. 454-457, 18-21 Oct. 2011 discusses the application of switched excitation for ultrasound generation. It is noted that switched excitation introduces undesirable harmonics into the signal compared to analogue signals. The reduction of harmonics through the addition of intermediate switching levels and control of the switching timing is proposed, and in particular two, three, five and nine level switched excitation signals are described, and their harmonic performance simulated and experimentally verified. However, no detail is given regarding how the multi-level switched excitation signals are generated.